This invention relates to an electrolytic treatment apparatus using an improved auxiliary counter electrode, and more particularly, to an electrolytic treatment apparatus suitable for graining a metal web by an alternating electric current, in particular, for graining an aluminum support for a printing plate suitable for offset printing.
As a support for a printing plate, in particular, a lithographic printing plate, an aluminum plate is used. For various requirements of users, various kinds of aluminum plates of from an almost pure aluminum plate to an aluminum plate containing manganese for increasing the strength are employed.
For using such an aluminum plate as a support for a printing plate, the aluminum plate is required to have a proper adhesive property with a light-sensitive material coated thereon and a water-holding property.
For the purpose, it is necessary to grain the surface of the aluminum plate such that the aluminum plate has the uniformly and precisely grained surface. Since the graining treatment gives remarkable influences on the printing performance of the printing plate, such as a staining performance of the printing plate, etc., the quality of the graining treatment is an important factor for producing printing plate in the case of actually performing printing.
As the graining process of an aluminum support for printing plate, there are a mechanical graining process, an electrochemical graining process, and a combination of these processes.
As the mechanical graining process, there are, for example, a ball graining process, a wire graining process, a brush graining process, and a liquid honing, etc. Also, as the electrochemical graining process, an alternating electrolytic etching process is generally employed, and in this case, an ordinary sine wave alternating current or a specific alternating current such as square wave alternating current, etc. is used. Also, as a pretreatment for the electrochemical graining treatment, an etching treatment by an aqueous sodium hydroxide solution may be employed.
In the alternating electrolytic etching process, there is a problem that a counter electrode composed of carbon or a metal is very liable to be deteriorated, differently from the phenomenon caused by a direct current. For example, when a counter electrode composed of carbon is used, the reaction of oxidation and reduction is repeatedly applied to the counter electrode whenever the polarity of the counter electrode is changed, to greatly deteriorate the binder for the counter electrode, whereby it is very difficult to use the electrode stably for a long period of time.
For solving the problem, an electrolytic treatment apparatus is disclosed in Japanese Patent Application Examined Publication No. 48596/86. In this apparatus, a circuit for an auxiliary counter electrode is connected, in parallel, to a circuit for a main counter electrode, and a diode or a mechanism of performing a diode-like action for controlling an anode electric current in the main counter electrode is provided in the circuit for the auxiliary counter electrode.
However, in the auxiliary counter electrode equipped with the diode-like mechanism, only an anode current passes through the auxiliary counter electrode, whereby the electrode is greatly deteriorated, the effect of forming the auxiliary counter electrode is reduced, and the deterioration of the auxiliary electrode becomes larger in cost than that of the main counter electrode.
Recently, a material such as platinum, iridium oxide, etc., has been practically used for anode current but the cost for exchanging such an electrode is very high.
As described above, the cost by the deterioration of the auxiliary counter electrodes in an electrolytic treatment apparatus is recently increased with the increase of mass production.
The following electrolytic treatment process, capable of preventing the occurrence of the deterioration of an auxiliary counter electrode without reducing the conventional electrolytic treatment faculty, is proposed in Japanese Patent Application No. 138479/88.
That is, the process is an electrolytic treatment process of applying an electrochemical treatment by supplying an alternating current between a material to be treated and a counter electrode in an electrolyte, wherein the electrolytic treatment being carried out by forming the counter electrode by a main counter electrode and an auxiliary counter electrode, connecting a circuit for the aforesaid auxiliary counter electrode, in parallel, to a circuit connected to the main counter electrode, providing a diode or a mechanism performing a diode-like action in the circuit for the auxiliary counter electrode for controlling an anode current in the main counter electrode, selecting the electrolytic condition in the auxiliary counter electrode t differ from that in the main counter electrode. For example, the concentration and temperature of the electrolyte at the side of the auxiliary counter electrode are selected to become lower than those at the side of the main counter electrode.
However, in the aforesaid process, an expensive material such as platinum, etc. is used as the auxiliary counter electrode. Thus, an inexpensive and more tough substituent for the expensive material has been desired.